This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-117035, filed Apr. 18, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a method of cleaning a shadow mask, which is used in a cathode ray tube, by an ultrasonic and an apparatus for cleaning the shadow mask.
In general, a cathode ray tube used in a color television set or the like comprises a vacuum envelope including a panel and a funnel. A phosphor screen formed on the inner surface of the panel includes blue (B), green (G) and red (R) phosphor layers and black layers formed therebetween. A shadow mask is arranged inside the panel and opposite to the phosphor screen.
The shadow mask comprises a mask body, which has a mask surface where a number of electron beam passage apertures are formed and a skirt portion at the peripheral edge of the mask surface, and a mask frame, which is welded to the skirt portion of the mask body. Holders are welded to the respective corners of the mask frame. Further, panel pins provided on the inner wall of the panel are engaged in installation holes formed in the holder, respectively, thereby supporting the shadow mask at a predetermined position opposing the inner surface of the panel.
In the assembling process of the shadow mask, the mask body is formed into a predetermined shape by press-molding a raw material plate having a large number of electron beam passage apertures formed therein in advance. The formed mask body is cleaned and, then, subjected to a blackening treatment to have the surface covered with a blackening film consisting of an oxide film. The blackening film functions to prevent rust and reflection.
The mask frame of the shadow mask is also subjected to cleaning and process of blackening, after the press molding. The holder is welded to each corner or each side of the mask frame. This mask frame is welded to the outer surface of the skirt portion of the mask body at a plurality of positions. In general, the mask frame is formed of iron, and the mask body is formed of iron or Invar. Spot welding utilizing the resistance welding is adopted for the welding between the mask body and the mask frame.
The welding apparatus used for the spot welding comprises a pair of electrodes, i.e., a pressing-side electrode and a back electrode. In the welding step, the junction between the skirt portion of the shadow mask and the mask frame is held with a predetermined pressure between the pressing-side electrode and the back electrode. Under this state, voltage is applied between these two electrodes, with the result that the skirt portion and the mask frame are subjected to a resistance welding. To be more specific, if an electric current flows between the pressing-side electrode and the back electrode, the skirt portion and the mask frame are welded so as to form a welded portion called nugget.
It should be noted that, in the welding step, the blackening film poor in conductivity, which is formed on the surfaces of the mask body and the mask frame, is present between the pressing-side electrode and the back electrode. As a result, splashes are generated when the blackening film is broken and when the metals are welded to each other. The splashes thus generated are scattered on the mask surface so as to cause clogging of the electron beam passage apertures.
To be more specific, the mask body has a thickness of 0.1 to 0.25 mm, and a plurality of amphitheatric circular or rectangular openings each having a diameter of 100 to 200 xcexcm are bored in the front and back surfaces of the mask body. Each of these openings has a larger diameter on the side of the surface facing the phosphor screen of the panel and a smaller diameter on the side of the surface facing the electron gun. Each of the electron passage apertures is defined by a pair of larger and smaller diameter openings. Further, in the welding step, splashes scattered from the welding portion onto the mask surface easily enter the smaller diameter openings or the larger diameter openings so as to causes clogging.
Particularly, since the welding points are positioned in the peripheral portion of the shadow mask, the clogging caused by the splashes tends to take place in the peripheral portion of the mask body. Also, the foreign matters such as the splashes tend to be accumulated in the clearance between the shadow mask and the mask frame.
On the other hand, in the manufacturing process of the cathode ray tube, the shadow mask is used for the formation of a phosphor screen. To be more specific, the phosphor screen is formed by exposing the phosphors of the three colors of blue, green and red to the light passing through the apertures of the shadow mask arranged to face the panel. It follows that, if the apertures of the mask body are clogged by the splashes, dust or other foreign matters, it is impossible to expose the phosphors to light in a desired pattern, resulting in formation of the phosphor screen having defects.
Under the circumstances, it is supposed to clean the shadow mask after the welding step so as to remove the foreign matters clogging the apertures. An ultrasonic cleaning within water may be employed for the cleaning.
In the ultrasonic cleaning treatment noted above, the shadow mask is held horizontal or vertical within water, and the shadow mask is irradiated with the ultrasonic wave oscillated from an ultrasonic generator so as to remove the foreign matters. For example, in the case of irradiating the ultrasonic wave from the side of the mask frame of the shadow mask, i.e., from the side of the smaller diameter openings on the inner surface of the mask body, the foreign matters clogging the larger diameter openings are removed by the ultrasonic wave passing through the smaller diameter openings.
However, the ultrasonic wave passing through the apertures of the shadow mask and the ultrasonic wave passing outside the shadow mask are not effectively utilized by simply oscillating the ultrasonic wave from the ultrasonic generator toward the shadow mask as described above. It is difficult to achieve an effective irradiation of the ultrasonic wave in that portion of the shadow mask which is difficult to be irradiated with the ultrasonic wave, giving rise to the problem that the shadow mask cannot be cleaned sufficiently.
Particularly, where the shadow mask is irradiated with the ultrasonic wave coming from the side of the smaller diameter openings on the inner surface of the mask body, the mask frame, which is obstructive, causes the peripheral portion of the mask body to be unlikely to be irradiated with the ultrasonic wave, resulting in failure to clean sufficiently the entire mask body.
The present invention has been contrived in consideration of the above circumstances, and its object is to provide a method of cleaning a shadow mask without fail and a cleaning apparatus of the shadow mask.
In order to achieve the above object, a cleaning method according to the present invention comprises:
arranging a shadow mask including a mask body having electron beam passage apertures formed therein and a mask frame fixed to the peripheral portion of the mask body within an ultrasonic wave transmitting medium;
oscillating an ultrasonic wave from an ultrasonic generator toward the shadow mask; and
allowing an ultrasonic wave reflector to reflect at least partially the ultrasonic wave oscillated from the ultrasonic generator toward the shadow mask.
A cleaning apparatus according to the present invention comprises:
a holding section for holding a shadow mask within an ultrasonic wave transmitting medium;
an ultrasonic generator for oscillating an ultrasonic wave toward the shadow mask; and
an ultrasonic wave reflector for reflecting at least partially the ultrasonic wave oscillated from the ultrasonic generator toward the shadow mask.
According to the cleaning method and the cleaning apparatus of the present invention, an ultrasonic wave is oscillated from an ultrasonic generator toward a shadow mask so as to permit the ultrasonic wave to remove the foreign matters attached to the shadow mask, thereby cleaning the shadow mask. Also, the ultrasonic wave oscillated from the ultrasonic generator is reflected at least partially toward the shadow mask so as to effectively utilize the ultrasonic wave and, thus, to wash the shadow mask without fail.
Also, according to the present invention, the ultrasonic wave reflector reflects partially the ultrasonic wave oscillated from the ultrasonic generator toward the peripheral portion of the mask body, thereby permitting the peripheral portion of the mask body, which is unlikely to be irradiated directly with the ultrasonic wave oscillated from the ultrasonic generator, to be irradiated with the ultrasonic wave sufficiently, thereby cleaning the shadow mask without fail.
Also, according to the present invention, the ultrasonic wave reflector reflects the ultrasonic wave passing through the electron beam passage apertures of the shadow mask or reflects directly the ultrasonic wave oscillated from the ultrasonic generator so as to permit the shadow mask to be irradiated with the ultrasonic wave, thereby effectively utilizing the ultrasonic wave.
Further, according to the cleaning method and the cleaning apparatus of the present invention, the relative positions of the shadow mask and the ultrasonic wave reflector are changed so as to change the position of the shadow mask irradiated with the ultrasonic wave reflected from the ultrasonic wave reflector. As a result, a wide range of the shadow mask is irradiated with the ultrasonic wave and the ultrasonic wave irradiation is intensified or weakened at a desired area of the shadow mask so as to efficiently clean the shadow mask.
It follows that the present invention provides a cleaning method and a cleaning apparatus that can sufficiently clean the shadow mask, and also provides a high quality shadow mask free from the difficulties such as the clogging.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.